eigen: Passing non-referencable array to `py::cast(my_array, rvp::reference_internal, parent)` silently fails

I accidentally ran into an issue where our code had some confusing overloads; GetX(state) returned a reference to a vector, while GetX(state, specifier) returned a copy of a vector (due to non-contiguous sub-indices).

I had accidentally bound both as reference methods, like so:

cls
  .def("GetX",
    [](const State& state) {
      return py::cast(
          GetX(state), reference_internal, py::cast(&state, reference));
    })
  .def("GetX",
    [](const State& state, const Specifier& spec) {
      return py::cast(
          GetX(state, spec), reference_internal, py::cast(&state, reference));
    });

When testing, GetX(state) would return the values I wanted, but GetX(state, specifier) would return all zeros. Once I realized it was just a copy, I was able to fix this issue.

This seems like something that should be easily caught before it gets to eigen_array_cast, though it would require having knowledge of whether or not the type being passed in is (a) non-Map (which I would assume falls to the nominal dense type_caster) and (b) a pointer type, which should be caught between the indirection from cast([const] Type&[&], ...) to cast_impl(...).

Trace the cast_impl and eigen_array_cast entry points, including the nominal dense type_caster path described in the issue. Reproduce the non-referencable array case and inspect how reference_internal is handled between the reference overloads and eigen_array_cast. Done means the invalid reference case no longer silently produces incorrect values and has regression coverage.